Technology Series - Design of Gear Box Cover Casing using SolidWorks Part II Continued...

Revised Design with Deflection Plot

One of the many designs considered to increase the stiffness is shown above. The stiffness in the axial direction of the main gear shaft is influenced by the bending stiffness of the casing wall. It is important to bear in mind, at this stage, the manufacturing process adopted for the stated part. Gating, risering, fill time, solidification time, shrinkage distribution, warpage, sink marks and pre-stresses arising out of non-uniform cooling are some of the aspects that need focus to arrive at feasible stiffner pattern and distribution. Stiffening methodology is driven and prioritized by the aforesaid considerations. Criticality matrix of influencing parameters and their effects can be evolved to drive such design decisions.

Revised design yielded a stiffness value of 331,50,000 N/mm. It is important to bear in mind that the calculations for the initial and revised designs are performed using same loads, boundary conditions, element type, element size among other parameters.

Note: Though the revised design has met the minimum stiffness value 3 times over, the design can be revisited to reduce the stiffness to required levels with cost considerations driving the same.

Manufacturing and Assembly considerations

Drawings issued for the gearbox casing have to reflect design specification while addressing manufacturing and assembly considerations. Choice of datum are vital to achieve design specifications for least costs. Functional requirements necessitate using the gearbox casing mating face and main-gear shaft axis as primary and secondary datum references. The tertiary datum, required to prevent rotation about main-gear shaft axis could be a flat face or another axis ! Selection of Latter feature helps establish datums based on parameters that are under manufacturer's control. However, locating pads are required as primary features that help define the stated datum and datum features of size.

Choice of locations and orientations of Locating pads are important to meet the design specifications with least rejection rates and cost. Flatness of primary datum (mating face) is important to control oil leakage. During machining, if the casing deflects due to clamping forces, achieving critical flatness tolerances would be near impossible resulting in increased cost and higher rejection rates. Herein comes the stiffness of the casing to achieve stated tolerances ! This consideration would have escaped normal design practice unless the criticality matrix approach is adopted for the design process as a whole. Another functional influence is the location of the pattern of holes used to fasten the gearbox casing along with the tightening torque employed. Due to pre-torque used to tighten fasteners, the cover frequencies could change due to induced stiffening !

Location of hole pattern needs to consider eveness of the clamping pressure across the mating face, subjected to working loads during operation. Lesser the number of holes, lesser the number of fasteners. This results in lesser cost of fasteners and accompanying washers, assembly time, spares packing list among others.

Though there are many more considerations, exemplified descriptions, provided above, are intended to address the functional priorities that need higher levels of attention for trouble-free operation on site.

Core-cavity analysis in SolidWorks provides good feedback on manufacturability aspecs of casting design. DFMXpress also helps understand the evaluation of manufacturing considerations a priori.

Draft Analysis of Gear Box Casing

Gear Box Cover Partial Drawing

Gear Box Casing Drawing (Partial) using DimXpert

Tolerance allocation for meeting Assembly Stack-up requirements:

Feature control frames form the heart of the Tolerance analysis system. DimXpert enables the design engineers to rapidly develop drawings conforming to ASME Y 14.5 GD & T Standard. This ensure correctness and completeness of the drawings produced.

Centre distance between shafts form an important part of the gear box assembly. Based on position tolerances defined on the drawing, extremum positions of shafts can be arrived at, based on Feature control frames and material modifiers used. Size tolerances and position/ orientation tolerances can be used to construct 1-Dimensional tolerance stack-up analysis required for this kind of application.

Gear play due to backlash, stack-up, deflections (due to self-weight, operational loads), thermal expansion (if temperature of operation is appreciable) need to be weighed carefully to arrive at overall stack-up effects and influence.

After establishing the tolerance framework for individual components, assembly stack-up analysis can be performed using TolAnalyst in a simple manner. Advanced tolerance calculations including process capabilities and Six Sigma approach can be performed with cost optimization using SigmundWorks inside SolidWorks.